The field of the present invention relates generally to the automatic control of the transient and steady-state characteristics of power switching circuits and devices, and more specifically to the automatic control of power switching circuits and devices having bipolar and field effect transistors.
A VMOS/bipolar transistor dual-triggered power switch is discussed in the U.S. patent application of Baker, Ser. No. 40,989, which is assigned to the common assignee herewith. The power dissipation of the VMOS/bipolar dual-triggered switch is minimized by controlling the turn-on and turn-off characteristics of a highpower bipolar transistor which is included in the switch and by maintaining the power output transistor in a controlled state of saturation during steady-state on operation. The steady-state current supplied to the base of the power transistor is preferably controlled by a VMOS device, however certain parameters of VMOS and bipolar transistors prevent precise control of such base current. These parameters include the transconductance of the VMOS device, the base-emitter resistance of the power output transistor, and the base-emitter voltage of the power output transistor, each of which varies from device to device and with temperature. The disadvantage created by imprecise control of the base current for the power transistor is that the depth of saturation of the power transistor cannot be precisely set. Accordingly, power is lost because of excessive steady-state current applied to the base of the power output transistor and because of imprecise and unrepeatable control of the turn-off characteristics of the output power transistor.
Techniques are known in the art for controlling current flow through transistors. Thus, it is generally known in the art to sense the amount of current flowing through a resistor and to operate an electronic switch when the current exceeds a predetermined value. Such an apparatus is disclosed in Roveti, U.S. Pat. No. 3,916,220. The Roveti apparatus, however, is not used in a combination VMOS/bipolar dual-triggered switch and does not teach how to minimize steady-state power dissipation by precisely controlling the level of saturation of a power transistor. Furthermore, Roveti does not teach how to improve the precision and speed of switching time control by controlling base current to the power output transistor. Accordingly, there is a need in the art for an improved power switching device.